I'm 20 and my periods have been coming lighter and shorter?

Life changes? Have you gained/lost significant amount of weight recently? Are you involved in athletics? Are you on birth control? Have you been under a lot of stress lately. These are some of the factors that can affect your periods. Best to see fp or gynecologist to determine the cause.
Can be normal. Having lighter periods over time can be a normal thing. Birth control pills can also cause you to have very light periods as a nice side effect.
May be normal. Are you on a form of hormone contraception where this is normal? A significant change in weight can be a factor. Other medications may also cause this. If cycles are regular and predictable, just lighter, it may be a normal life change for you. If in doubt or worried, further evaluation will help.